1. Field of the Invention
The present disclosure relates to a method of predicting a corrosion rate of a steam generator tube using eddy current test, and more particularly, to a method of performing eddy current test using a motorized rotating pancake coil (MRPC) and predicting a corrosion rate of a steam generator tube from an MRPC noise value.
2. Description of the Related Art
The integrity of nuclear power plants is influenced by corrosion damages and mechanical erosions of their consisting materials in coolant system. Particularly, steam generator tube, which occupies about 70% of primary surface area and is exposed simultaneously to primary and secondary coolants, is a crucial structural component to secure the integrity of nuclear power plant. The corrosion release of metal ions or corrosion products from steam generator tube which causes the increase of radiation dose in primary coolant system or axial offset anomaly (AOA) becomes more important because the corrosion damages of steam generator tubes such as stress corrosion cracking and intergranular attack are still reported since the steam generator tubing material is replaced from Alloy 600 to Alloy 690. The corrosion release rate can be predicted indirectly through in-situ monitoring of dose amount for 58Co and 60Co as major radiation sources during operation of nuclear power plant. Then, it has been reported that corrosion release properties of the steam generator tube are influenced by various factors including material intrinsic properties such as its chemical composition or surface property and manufacturing methods. Therefore, to employ the steam generator tubes having excellent corrosion release resistance is very important from manufacturing step prior to any others and the establishment of an innovative prediction method for corrosion release properties at a selection step of materials or a manufacturing step of steam generator is needed.
The eddy current test (ECT) is typically used for evaluating in-service integrity of the steam generator tube. The eddy current signal is typically influenced by manufacturing quality of the steam generator tube, corrosion products and foreign objects deposited on the steam generator tube surface, or its adjacent structures. Thus, to reduce the influence factors on the eddy current signal is very important to improve a probability of detection and it indicates that the manufacturing quality of steam generator tube becomes more significant. However, a bobbin signal-to-noise (S/N) ratio is only required currently at the manufacturing step of the steam generator for improving the probability of detection during in-service inspection. According to the practical results, it has been reported that the radiation doses of 58CO and 60CO were varied by producing factors of steam generator tubes such as manufacturing periods or type of process, although the steam generator tube satisfies the requirement for the bobbin S/N ratio. In addition, criterion on the corrosion release property is not yet established as a requirement and there is no method capable of predicting them in advance.
Meanwhile, as an existing technology related to the eddy current testing of the steam generator tube, Korean patent No. 10-1083215 discloses an eddy current testing method using an MRPC probe for inner diameter circumferential crack of a steam generator tube. In detail, it discloses the ECT method using an MRPC probe for inner diameter circumferential crack of the steam generator tube including: (a) obtaining the eddy current signals with varying frequencies of the MRPC inspection on the steam generator tubes; (b) analyzing signals obtained through the ECT under different frequencies and inspecting the inner diameter circumferential crack; (c) drawing a calibration curve for the inner diameter circumferential crack and selecting the eddy current signal at any one frequency among the ECT signals at several frequencies; (d) measuring a depth from a starting point to a ending point of the crack with predetermined interval; and (e) calculating a percent degraded area (PDA) considering a crack shape and length. However, the prediction of corrosion and release properties becomes more significant than the measurement of its corrosion damages such as stress corrosion cracking (SCC) because they are still reported for Alloy 690 dislike Alloy 600 as a typical steam generator tube material.
Therefore, the inventors found that MRPC noise signals have a relationship with the corrosion rate of the steam generator tube when they were performing the study on the prediction method for corrosion and release properties of steam generator tube and then completed the present invention possible to predict the corrosion and release properties of steam generator tubes at the component selection step.